Analog-to-digital conversion method and device

ABSTRACT

A method for analog/digital conversion of a radiocommunication signal using an analog analog/digital converter preceded by variable gain amplifier, wherein the frequency band of the converted signal contains at least one useful channel, comprises steps for controlling the value of the gain of the amplifier in such a way that the power in the frequency band of the converted signal is less than a limit saturation value of the analog/digital converter. However, the power in the useful channel is substantially equal to a mean power level having a first predetermined value in the case of static propagation in the useful channel or a second predetermined value, different from said first predetermined value, in the case of dynamic propagation in the useful channel.

[0001] The present invention relates to a method of analog/digital conversion of a radiocommunication signal, with the aid of an analog/digital converter preceded by a variable-gain amplifier. The converted signal has a determined frequency band containing at least one useful channel, in general from among several different channels. A useful channel is the frequency band of minimum width making it possible to recover a useful signal.

[0002] The invention finds applications in radiofrequency receivers, such as may be found for example in the fixed equipment (base stations) or the mobile equipment (portable terminals) of radiocommunication systems. In such an application, the signal considered is for example a radiofrequency signal such as a phase-modulated and/or amplitude-modulated carrier, or a signal resulting from the transposition of such a signal to an intermediate frequency or to baseband. The useful signal contains information coding voice and/or data and/or signaling information.

[0003] In state of the art receivers, the frequency band of the converted signal corresponds to the useful channel. Accordingly, the radiofrequency signal received on a reception antenna is filtered, in general after transposition to an intermediate frequency or to baseband, by means of a filter disposed in the reception chain upstream of the analog/digital converter. In this way, the latter converts only the energy of the signal in the useful channel. When the receiver is a multichannel receiver, channel selection means, comprising a selective filter, make it possible to select the useful channel from among a plurality of different channels. In a manner known per se, a variable-gain amplifier can precede the analog/digital converter in the reception chain, so as to tailor the power level in the useful channel to the input dynamic swing in power of the converter. This is achieved by virtue of appropriate automatic gain control means.

[0004] Attempts are currently being made to dispose the analog/digital converter nearer to the reception antenna, and in particular upstream of the channel selection means. Hence, the signal converted by the analog/digital converter has a determined frequency band containing at least one useful channel, in general from among several different channels. The width of the frequency band of the converted signal, or converted band, is then greater than that of the useful channel. This converted band is determined by the (analog) filters disposed upstream of the converter. The channel selection means, disposed downstream of the analog/digital converter in the reception channel, then comprise a mixer and one or more digital filters for selecting the useful channel, before digital demodulation and decoding.

[0005] However, the power of the radiofrequency signal received on the reception antenna varies over time. These variations may be due to the appearance or the disappearance of obstacles between the transmitter and the receiver, to the appearance or the disappearance of other signals in the frequency band occupied by the signal, or to “fading” when there is a relative motion of the receiver with respect to one of the transmitters. One speaks of propagation of dynamic type in a channel when there is a relative motion of the receiver with respect to the corresponding transmitter, and of propagation of static type in the converse case. Fading is considerable when the Doppler frequency of f₀×v/c is considerable, where fo is the central frequency of the channel, v is the relative speed of the receiver with respect to the transmitter and c is the speed of light. It is noted that when a signal is situated in a “fading hole” its power may become very small. The decrease in power of the signal in a “fading hole” is of short duration. In fact, the shorter the duration of the “fading hole”, the smaller the power of the signal in the “fading hole”.

[0006] Moreover, the input dynamic swing in power of the analog/digital converter is limited above by an upper limit value beyond which the converter is saturated, and below by a lower limit value beneath which the signal can no longer be distinguished from the noise introduced by the converter. One speaks of saturation value to designate said upper limit value, and of noise floor to designate said lower limit value. By convention, the desired power levels and the mean power values indicated subsequently in this document may be expressed in decibels (dB) with respect to the noise floor of the converter.

[0007] Furthermore, any radiocommunication system complies with specifications which determine the sensitivity and the rejection of the system with regard to the useful channel, as a function of the type of propagation in this channel. The sensitivity of the system corresponds to the minimum power level of the signal in the useful channel (signal-to-noise ratio), at which the system must still operate. The rejection of the system corresponds to the maximum power level which must be tolerated by the system inside the converted band, in the channels neighboring the useful channel. The sensitivity in the static case is less than the sensitivity in the dynamic case, and the rejection in the static case is greater than the rejection in the dynamic case. In one example, the dynamic sensitivity is equal to 15 dB above the noise floor of the converter, and the dynamic rejection is equal to 45 dB. Moreover, the static sensitivity is equal to 7 dB above the noise floor of the converter and the static rejection is equal to 70 dB. The input dynamic swing in power of the converter which is necessary is therefore equal to 60 dB in the dynamic case, to 77 dB in the static case, and hence to 85 dB to cover both the static case and the dynamic case if the useful signal level is fixed at the same level in both cases. At a rate of 6 dB per digit, a converter operating on 15 bits at output is therefore necessary.

[0008] An object of the present invention is to propose a method and a device which make it possible to use a converter operating on a smaller number of bits at output, and which is therefore less expensive.

[0009] Accordingly, the invention proposes a method of analog/digital conversion of a radiocommunication signal with the aid of an analog/digital converter preceded by a variable-gain amplifier, the frequency band of the converted signal containing at least one useful channel, consisting in controlling the value of the gain of the amplifier in such a way that the power in the frequency band of the converted signal is less than a limit saturation value of the analog/digital converter and that the power in the useful channel is substantially equal to a mean power level having a first predetermined value in the case of static propagation in the useful channel or a second predetermined value, different from said first predetermined value, in the case of dynamic propagation in the useful channel.

[0010] By distinguishing the static case from the dynamic case, it is possible to decrease the input dynamic swing in power of the converter. Thus, returning to the values of the example above, the values of the gain of the amplifier can be controlled in such a way that the power in the useful channel is substantially equal to 7 dB above the noise floor of the converter in the static case, and to 15 dB above the noise floor of the converter in the dynamic case. In this way, the input dynamic swing in power of the converter which is necessary to cover both cases is equal to 77 dB. A converter operating on 13 bits at output is therefore sufficient, and is much less expensive than a converter operating on 15 bits at output.

[0011] The invention also proposes a device for analog/digital conversion of a radiocommunication signal whose frequency band contains at least one useful channel, comprising an analog/digital converter preceded by a variable-gain amplifier, and means for controlling the value of the gain of the amplifier in such a way that the power in the frequency band of the converted signal is less than a limit saturation value of the analog/digital converter and that the power in the useful channel is substantially equal to a desired mean power level in the useful channel having a first predetermined value in the case of static propagation in the useful channel or a second predetermined value, different from said first predetermined value, in the case of dynamic propagation in the useful channel.

[0012] The invention further proposes a radiofrequency radiocommunication receiver incorporating such a device.

[0013] Other characteristics and advantages of the invention will become further apparent on reading the description which follows. The latter is purely illustrative and should be read in conjunction with the appended Figures, wherein:

[0014]FIG. 1 is the simplified diagram of a radiofrequency radiocommunication receiver incorporating a device according to the invention;

[0015]FIG. 2 is the diagram of a unit for measuring the power in the converted band or in the useful channel;

[0016]FIG. 3 is a curve illustrating the profile over time of the power of the signal in a useful channel;

[0017]FIG. 4 is a flow chart of the steps of the method according to the invention;

[0018]FIG. 5 and FIG. 6 are flow charts detailing substeps respectively of a step of determining the type of propagation in the useful channel and a step of determining a “fading hole” of the signal in the useful channel.

[0019] Represented in FIG. 1 is the diagram of a radiofrequency receiver incorporating a device according to the invention. The receiver comprises a reception antenna 10 linked to the input of a radiofrequency amplifier 11 which outputs a radiofrequency signal RF. This is for example a phase-modulated and/or amplitude-modulated signal. In one example, the spectrum of the RF signal lies in the 380-500 MHz band. It comprises several distinct channels which are, for example, all of the same width.

[0020] The RF signal is carried to a first input of a first mixer 12 a. A second input of the mixer 12 a receives a signal at a frequency f_(LO1) less than the frequency of the RF signal, delivered by a first local oscillator 13 a. In one example, the frequency f_(LO1) is equal to 154 MHz. The mixer 12 a outputs a signal IF1 which corresponds to the RF signal transposed to the intermediate frequency f_(LO1). The signal IF1 is filtered by means of a first bandpass filter 14 a. The output of the filter 14 a is linked to a first input of a second mixer 12 b. A second input of the mixer 12 b receives a signal at a frequency f_(LO2) less than the frequency f_(LO1) of the signal IF1, delivered by a second local oscillator 13 b. In one example, the frequency f_(LO2) is equal to 500 KHz. The mixer 12 a outputs a signal IF2 which corresponds to the RF signal transposed to the intermediate frequency f_(LO2). The signal IF2 is filtered by means of a second bandpass filter 14 b. The signal IF2 thus filtered corresponds to the radiocommunication signal S according to the invention.

[0021] The device according to the invention comprises a variable-gain amplifier 15 whose input is linked to the output of the filter 14 b so as to receive the signal S, and whose output delivers the amplified signal S. The device further comprises an analog/digital converter 16 whose input is linked to the output of the amplifier 15 so as to receive the amplified signal S and whose output delivers discrete instantaneous values or samples Sn of this signal. The sampling frequency f_(e) of the converter 16 is for example equal to 2 MHz, complying with the Shannon condition.

[0022] Specifically, by reason of the passband of the bandpass filters 14 a and 14 b, the frequency band of the signal S, called the converted band, is around 150 KHz wide. Stated otherwise, it is assumed that the rejection, in terms of power, is big enough outside of this band for it to be possible to neglect the spectral components of the signal S outside of this band. Moreover it is centered on the frequency 500 KHz. In one example, a useful channel lying in the converted band is considered to be centered on the frequency 450 KHz. The converted band comprising several channels such as the useful channel, the bandwidth of this channel is less than that of the converted band.

[0023] The device comprises a unit 18 for measuring the power in the converted band, whose input receives the samples Sn, possibly but not necessarily via a digital filter 17 whose passband corresponds to the width of the converted band. The device also comprises a unit 20 for measuring the power in the useful channel, whose input receives the samples Sn via a channel selection module 19. The module 19 comprises means of digital transposition, for transposing the useful channel to baseband. In the example, these means comprise a digital mixer making it possible to transpose the converted band in such a way that it is centered on 50 KHz. The useful channel is then centered on 0 Hz. The module 19 also comprises digital means of low-pass filtering making it possible to isolate the components of the useful signal. The samples S′n output by the module 19 are also transmitted to the downstream part 23 of the radiofrequency receiver, which here is represented overall by a box. This downstream part 23 comprises in particular the means of demodulation and of decoding of the useful signal, which make it possible to extract the data transmitted in the useful signal.

[0024] The units 18 and 20 produce respectively values P_(B) of the power in the converted band and values P_(C) of the power in the useful channel, which are provided at the input of a management unit 21 of the device. Preferably, the device comprises, for example, in the power measuring units 18 and 20, means for compensating for the difference of delay in the transmission of the values P_(B) and P_(C) which is due to the difference of the paths taken. This makes it possible to deliver at the input of the unit 21 values of the power in the converted band and in the useful channel referring to identical samples Sn. The management unit 21 comprises means for implementing a method according to the invention, and outputs a control signal for an automatic gain control module 22. The module 22 produces an analog signal which is carried to a gain control input of the variable-gain amplifier 15, so as to control the gain of this amplifier in the manner which will now be described.

[0025] The gain of the amplifier is preferably controlled as a function of values of the mean power in the converted band and/or in the useful channel, in order for the device not to be too sensitive to the slight fluctuations in the power of the signal received. Specifically, taking account of values of the instantaneous power would give rise to changes in the gain of the amplifier which could prove to be inopportune in the sense that they might destabilize the reception chain. The values of the mean power are calculated over a determined time window. The larger this time window, the less sensitive is the device to the fluctuations in the power of the signal received. From another point of view, the measurement of the mean power of the signal is then available only after the expiry of this time window. This lag may be penalizing in certain cases, in particular on switching on the receiver. Specifically, it causes a delay in the adjusting of the reception chain which takes place in a receiver initialization phase.

[0026] This is why, according to an advantageous characteristic of the invention, the units 18 and 20 each produce N series of values, denoted P₁n to P_(N)n in what follows, of the mean power respectively in the converted band (aforesaid values P_(B)) and in the useful channel (aforesaid values P_(C)), where N is an integer, the values of these series being calculated over increasing time windows of respective width. In one example, N is equal to 5.

[0027] Represented in FIG. 2 is the simplified diagram of a power measurement unit such as the aforesaid unit 18 or the unit 20. In what follows, the terms “input signal” used with reference to the units 18 and 20 designate the signal corresponding respectively to the series of samples Sn and S′n provided as input to the units 18 and 20, that is the samples of the signal in the converted band and in the useful channel respectively. For the unit 18, the series of values P₁n to P_(N)n correspond to the values of the power in the converted band which are indicated overall by the reference P_(B) in FIG. 1, while for the unit 20, the values P₁n to P_(N)n correspond to the values of the power in the useful channel which are indicated overall by the reference P_(C) in FIG. 1. The description of a power measurement unit which follows with reference to FIG. 2 relates to the example of the unit 18 receiving the series of samples Sn as input signal. Given that the unit 20 is identical to the unit 18, this description also holds, with the necessary alterations to the notation, for the unit 20 receiving the series of samples S′n as input signal.

[0028] The power measurement unit of FIG. 2 optionally comprises a subsampling module 101 which carries out a subsampling of the samples of the input signal, at a subsampling frequency f_(se) which is a submultiple of the sampling frequency f_(e). In one example, f_(se)=f_(e)/125, so that one sample Sn out of 125 is transmitted by the module 101. The values of the samples Sn are for example coded on p bits, where p is an integer.

[0029] The unit further comprises a module 102 for calculating instantaneous power receiving as input the series of values Sn. This module has the function of producing a series of values Pn of the instantaneous power of the input signal, from the series of values Sn. The values Sn being writable in the form of an imaginary number Sn=S_(I)n+i. S_(Q)n, where S_(I)n and S_(Q)n are real numbers and where i²=−1, the values Pn are obtained successively from the successive values Sn by performing for each the calculation Pn=S² _(I)n+S² _(Q)n. The values Pn are therefore coded on 2p bits.

[0030] The unit further comprises, according to the invention, N mean power calculation modules arranged in cascade, where N is an integer. Each of these modules, referenced 103 ₁ to 103 _(N) in FIG. 1, allows the continuous production of the series of values P₁n to P_(N)n respectively of the mean power of the input signal, calculated over increasing respective time windows directly or indirectly from the values Pn of the series of values of the instantaneous power of the input signal. The modules 103 ₁ to 103 _(N) are in what follows called modules for calculating mean power of level 1 to N respectively. These are synchronous modules.

[0031] The module 103 ₁ for calculating mean power of level 1 comprises a memory register 104 ₁, as well as a counter C₁ (not represented) counting up to N₁, where N₁ is an integer such that N₁≧2, (not represented) and means for resetting to zero the register 104 ₁ and the counter C₁ (also not represented) . It further comprises means of addition 105 ₁, a first input of which is coupled to the output of the circuit 102 for calculating instantaneous power so as to receive the values Pn of the instantaneous power of the input signal, a second input of which is coupled to an output of the register 104 ₁ so as to receive the current value stored in this register, and the output of which is coupled to the input of said register 104 ₁. With each reception of a new value Pn, the means of addition 105 ₁ produce a value equal to the. sum of said value Pn and of said current value stored in the register 104 ₁ this sum value being then stored in the register 104 ₁ in place of said current value. Stated otherwise, the hereinabove means of the module 103 ₁ form an accumulator register. Such a register is of very simple structure and requires little memory space, since the register 104 ₁ must have a length enabling it to store the result of the addition of N1 words of 2p bits, that is equal to 2p+N1 only.

[0032] The module 103 ₁ for calculating the mean power of level 1 outputs a series of values P₁n which are obtained successively for example by averaging over N₁ successive values Pn of the instantaneous power of the input signal. Preferably, this is an arithmetic mean, which is the simplest to implement since it requires just one complex step of division by N₁. Accordingly, the counter is incremented by one unit with each reception of a new value Pn and corresponding updating of the value stored in the register 104 ₁. When the counter reaches the value N₁ the value stored in the register 104 ₁ is divided by N1 to yield an arithmetic mean of the last N₁ successive values Pn of the instantaneous power of the input signal which were received at the input of the module 103 ₁. A value P₁n of the mean power of level 1 of the input signal is thus produced. Moreover, the value of the counter C₁ and the value stored in the register 104 ₁ are reset to zero. Preferably, the integer N₁ is an integer power of 2, that is there exists a nonzero integer k₁ such that N₁=2^(k) ^(₁) . This makes it possible to simplify the step of division by N₁ since it is then sufficient to eliminate the N₁ least significant bits of the value stored in the register 104 ₁ to produce the value P₁n.

[0033] Each circuit 103 _(j) for calculating the mean power of level j, where j is an index such that 2≦j≦N, produces a j-th series of values P_(j)n of the mean power of level j of the input signal from N_(j) values of the j−1-th series of values P_(j−1)n of the mean power of level j−1 of the input signal, where N_(j) is an integer such that N_(j)≧2. It is necessary to distinguish between the last module 103 _(N) (for which j=N) and the other modules 103 _(j) (for which 2≦j<N).

[0034] For the values of j such that 2≦j<N, the values of the j-th series of values P_(j)n of the mean power of the input signal are obtained successively by averaging over successive N_(j)-tuples of successive values of the j−1-th series of values P_(j−1)n of the mean power of level j−1 (immediately lower level) of the input signal. Preferably, this is an arithmetic mean, which is the simplest to implement since it requires few complex calculations.

[0035] Accordingly, each module 103 _(j) for calculating the mean power of level j can have the same structure as the module 103 ₁ for calculating the mean power of level 1 described hereinabove, with a counter C_(j) counting up to N_(j).

[0036] Nevertheless, in a preferred exemplary embodiment, each module 103 _(j) for calculating the mean power of level j comprises, in place of the memory register 104 ₁ of the module 103 ₁, a shift register 104 _(j) of length N_(j), that is comprising N_(j) elementary registers in series, as well as a counter C_(j) (not represented) counting up to N_(j) and means (also not represented) for resetting to zero the counter C_(j) and optionally the register 104 _(j). It further comprises means of addition 105 _(j) with N_(j) inputs which are linked respectively to the outputs of the N_(j) elementary registers 104_(j) so as to receive the N_(j) values stored in the shift register 104 _(j). The input of each module 103 _(j) is coupled to the output of the module 103 _(j−1) so as to receive the values P_(j−1)n and its output is coupled to the input of the module 103 _(j+1) so as to send it the values P_(j)n. Each time a value P_(j−n) is input into the shift register 104 _(j), the counter C_(j) is incremented by one unit. When N_(j) values P_(j−n) of the mean power of level j−1 (immediately lower level) have been input into the shift register 104 _(j), that is when C_(j)=N_(j), these N_(j) values are added together in the adder 105 _(j). The sum obtained is then divided by N_(j) to produce a value P_(j)n of the mean power of level j of the input signal. Moreover, the shift register 104 _(j) can be emptied of the values which it contains, by virtue of the abovementioned means for resetting to zero. Preferably, each integer N_(j) is an integer power of 2, that is there exists an integer k_(j) such that N_(j)=2^(k) ^(_(j)) . This simplifies the step of division by N_(j), as has been set forth previously.

[0037] The structure of the modules 103 _(j) for calculating mean power of level j for 2≦j<N thus enable them to keep in memory, in the shift register 104 _(j), the previous values of the mean power of level j−1. This history of the values of the mean power can thus be used at any instant, as will be described hereinbelow.

[0038] Let us now see the particular case of the last module 103 _(N). The values of the last series of values P_(N)n of the mean power of level N of the input signal are obtained successively by taking a sliding average over the successive K_(N)-tuples of the last N_(N) values of the N−1-th series of values P_(N−n) of the mean power of level N−1 (immediately lower level) of the input signal.

[0039] Accordingly, the module 103 _(N) for calculating the mean power of level N can have the same structure as the module 103 ₁ for calculating the mean power of level 1 described earlier, with a counter C_(N) counting up to N_(N), but which is not reset to zero after the calculation of each value P_(N)n.

[0040] Nevertheless, in a preferred exemplary embodiment, the module 103 _(N) for calculating the mean power of level N of the input signal comprises a shift register 104 _(N) of length N_(N), that is comprising N_(N) elementary registers in series, and an adder 105 _(N) with N_(N) inputs for receiving respectively the N_(N) values stored in the shift register 104 _(N), where N_(N) is an integer. The input of the module 103 _(N) is coupled to the output of the module 103 _(n−1) for calculating the mean power of level N−1 (immediately lower level) . With each input of a new value P_(N−1)n of the mean power of level N−1 of the input signal into the shift register 104 _(N), the values which are stored therein are shifted so that the oldest value P_(N−1)n stored in the shift register 104 _(N) is lost. A counter C_(N) (not represented) able to count up to N_(N) is incremented by one unit with each input of a new value P_(N−1)n into the shift register 104 _(N). Moreover, a sum of the N_(N) values newly stored in this register is calculated by virtue of the means of addition 105 _(N). As soon as the counter has reached the value N_(N) (C_(N)≧N_(N)), the sum thus obtained is divided by N_(N) to produce the value P_(N)n of the mean power of level N of the input signal, according to (preferably) an arithmetic mean. Preferably, the integer N_(N) is an integer power of 2, that is there exists an integer k_(N) such that N_(N)=2^(k)N, this simplifying the step of division by N_(N) as was set forth previously. This calculation produces a value P_(N)n of the mean power of level N of the input signal. The counter C_(N) is not reset to zero after the calculation of each value P_(N)n.

[0041] The modules 104 ₁ to 104 _(N) for calculating the mean power of level 1 to N respectively of the input signal are for example embodied in the form of hardware and/or software modules, for example in a microcontroller, an ASIC circuit, a DSP circuit, an FPGA circuit, or the like.

[0042] As will have been understood, the successive values Pn of the instantaneous power of the input signal which are delivered by the circuit 102 bring about the cascaded generation of the series of values P₁n to P_(N)n of the mean power of level 1 to N respectively of the input signal. Thus, a value P₁n of the mean power of level 1 is a value of the input signal mean power calculated over a time window of width equal to N₁ times an elementary duration separating two successive values Pn of the instantaneous power of the input signal. This elementary duration is equal to 1/fe×fse.

[0043] Likewise, a value P₂n of the mean power of level 2 is a value of the input signal mean power calculated over a time window of width equal to N₁×N₂ times this elementary duration. Expressed in a general manner, this signifies that a value P_(j)n of the mean power of level j of the input signal is a value of the signal S mean power calculated over a time window of width equal to N₁×N₂×. . . ×N_(j−1)×N_(j) times the duration separating two consecutive values Pn of the instantaneous power of the input signal. These time windows are therefore of respective increasing widths.

[0044] Thus, the more the level j of the mean power of the input signal increases, the more the small variations in the values of the input signal are masked in the value P_(j)n of this mean power of level j. Nevertheless, the smaller the level of this mean power, the more quickly are the values P_(j)n of the mean power available after the unit has been switched on. In one example, N is equal to five, N₁, N₄ and N₅ are equal to eight, and N₂ and N₃ are equal to two. Moreover, the values of the power in the converted band and/or the values of the power in the useful channel which are taken into account in an initialization phase are the values P₁n of mean power of level 1 which are calculated over a time window having a first determined width, while the values of the power in the converted band and/or the values of the power in the useful channel which are taken into account in a holding phase are the values P₅n of mean power of level 5 which are calculated over a time window having a second determined width, greater than said first determined width. Of course, values of the mean power of different levels may be taken into account for the power in the converted band and for the power in the useful channel. In one example, the initialization phase is considered to begin when the device is brought into service and to terminate as soon as a value P₅n of the mean power of level 5 in the useful channel is available. However, the device is brought back to the initialization phase and the memory registers and the counters Ci for i lying between 1 and N of the units 18 and 20 are reinitialized with each modification of the value of the gain of the amplifier 15. Moreover, and preferably, the values of the power in the frequency band of the converted signal and/or the values of the power in the useful channel which are taken into account are calculated on the basis of measurements of the instantaneous power after a first determined lag has elapsed after bringing into service or modifying a parameter of an analog part upstream of the analog/digital converter.

[0045] The values P_(B) of the power in the converted band and the values P_(C) of the power in the useful channel which are produced respectively in the units 18 and 20, are by nature decimal values on a linear scale. They are for example expressed in watts (W) or in milliwatts (mW) . Moreover, the mean power values are calculated from values in watts or in milliwatts. Nevertheless, it is advantageous to express them in decibel milliwatts (dBm), that is on a logarithmic scale. Specifically, the values of the gain of the amplifier 15 which can be controlled by the gain control signal delivered by the management unit 21 are generally expressed in dB. Likewise, the saturation value Psat and the noise floor Pmin of the converter 16 are generally expressed in dBm in the specifications. Also, the mean power level P_(C)o desired in the useful channel and the mean power level P_(B)o desired in the converted band are expressed in dBm. As was stated earlier, all these values can be expressed through a deviation in dB with respect to the value Pmin of the noise floor of the converter expressed in dBm. Likewise, various margins used in the comparison steps can be expressed in dB. It is indeed advantageous to deal with values expressed in dB, since operations of multiplication or of division on values expressed linearly are then performed by means of simpler operations of addition and of subtraction respectively.

[0046] This is why the values P_(B) of the power in the converted band and the values P_(C) of the power in the useful channel are converted into values in decibels by means of a predetermined conversion table stored for this purpose in the units 18 and 20 respectively. Such a table can take the form given by table I below. In the unit 20, there exists such a table for each type of power measurement of a channel (sampling frequency, channel filter used) lying in the frequency band of the radiocommunication signal S. In each of these tables, each column corresponds to one of the predetermined values which can be taken by the gain of the amplifier 15. These gain values go from a minimum value Gmin to a maximum value Gmax with a stepsize of for example 1 dB. Advantageously, the power values converted into decibels by means of the table are then independent of the current value of the gain G of the amplifier 15.

[0047] Likewise each row of the table corresponds to a measured power value going from a minimum value Pmin, which corresponds to the value of the noise floor of the converter possibly increased by a margin, to a maximum value P_(B)max (for the power in the converted band) or P_(C)max (for the power in the useful channel), with a stepsize ΔP of for example 0.5 dB. Each row of the table therefore corresponds to an index j such that the power value indicated in this row corresponds to Pmin+j.ΔP for j going from 0 to Np, where Np is an integer. TABLE I Gain Index Pa Gmin . . . . . . . . . . Gmax 0 Pmin X X X X X X X 1 Pmin + ΔP X X X X X X X 2 Pmin + 2.ΔP X X X X X X X : : X X X X X X X 3 Pmin + j.ΔP X X X X X X X : : X X X X X X X Np Pmin + NpΔP X X X X X X X

[0048] The conversion of any power value is carried out as follows. The value to be converted, expressed in watts or in milliwatts, is compared with the values from the column of the table corresponding to the current value of the gain G of the amplifier 15, which values are expressed in the same unit (W or mW). It is perhaps equal to one of these values or lies between two of these values contained in two adjacent rows of the table. If it is less than Pmin or greater than Pmin+NpΔp, it is set to Pmin or to Pmin+NpΔp, respectively. From this is then deduced the value of the index j corresponding to the row of the table whose value, for the relevant column, is closest to the value to be converted. The value of this index is saved in memory and is used to compare the power value with other power values converted in the same way. It was seen that Pmin corresponds to the zero value of the index j. In one example, Psat corresponds to the value Np of the index j. Stated otherwise, Psat=Pmin+Np.Δp. Index values (integers) are thus compared, instead of comparing values in watts or in milliwatts (decimal numbers). This is simpler. Moreover, the index values can be saved in place of the corresponding values in watts or in milliwatts. This occupies less room in memory.

[0049] In FIG. 3, the curve 50 represents an example of the profile versus time of the power P_(C) in a determined useful channel. Horizontal lines represent an interval around a desired predetermined mean power level P_(C)o which is regarded as satisfactory for the channel taking into account the input dynamic swing in power of the analog/digital converter 16. This interval is delimited by an acceptable maximum value P_(C)max and by an acceptable minimum value P_(C)min.

[0050] In case of static propagation in this channel, that is when the corresponding transmitter is fixed with respect to the receiver, the power P_(C) hardly varies over time. Its slight fluctuations are due only to parasitic glitches in the channel. In case of dynamic propagation in the channel, that is when the corresponding transmitter is moving with respect to the receiver, the power P_(C) varies somewhat more, and it may pass below the level P_(C)min as indicated for example by the reference 52 in FIG. 3, or above the level P_(C)max.

[0051] According to the invention, the desired mean power level P_(C)o, and possibly also the deviation between the acceptable minimum level P_(C)min and/or acceptable maximum level P_(C)max on the one hand and P_(C)o on the other hand, depend on the type of propagation in the useful channel. In one example, the values of P_(C)o, P_(C)min and P_(C)max are respectively equal to 4 dB, 7 dB and 10 dB above the noise floor Pmin of the analog/digital converter 16 (P_(C)min=Pmin+4 dB; P_(C)o=Pmin+7 dB; P_(C)max=Pmin+10 dB) in the static case, and respectively to 12 dB, 15 dB and 18 dB above Pmin (P_(C)min 32 Pmin+12 dB; P_(C)o=Pmin+15 dB; P_(C)max=Pmin+18 dB) in the dynamic case.

[0052] The type of propagation, static or dynamic, can be determined as a function of the state of the receiver (when the latter comprises means for detecting that it is moving), or of data received from the transmitter (when the latter comprises means for detecting and signaling that it is moving) . Nevertheless, the device according to the invention preferably comprises means for determining the type of propagation, static or dynamic, in the useful channel as a function of the history of the values of the power in the useful channel which are obtained in the absence of saturation of the analog/digital converter as will be explained in greater detail hereinbelow in conjunction with FIG. 5.

[0053] When the signal in the channel is in a “fading hole”, the power P_(C) in this channel may suddenly become less than the minimum level P_(C)min, as indicated for example by the references 51 and 53 in FIG. 3. However, this sudden decrease in the power in the useful channel is of short duration. Consequently, it may be preferable not to modify the gain of the variable-gain amplifier 15 (FIG. 1) on account of the power measurements in such a “fading hole”.

[0054] This is why the device comprises means for determining whether the signal in the useful channel is in a “fading hole”, and for modifying the gain of the amplifier 15, as appropriate, only if the signal in the useful channel is not in a “fading hole”. A “fading hole” is detected by the abrupt variation of the successive values of the power P_(C) in the channel, as will be explained in greater detail hereinbelow in conjunction with FIG. 6.

[0055] The manner of operation of the device, according to the method of the invention, is described hereinbelow with reference to the flow chart of FIG. 4.

[0056] When the receiver is switched on, the device operates according to an initialization phase. Subsequently, it operates according to a holding phase. The method of analog/digital conversion whose steps are represented on the flow chart of FIG. 4 and implemented both during the initialization phase and during the holding phase. It will be now be described in the case of the initialization phase. This description is, making the necessary alterations which will be pointed out, also valid for the implementation in the holding phase. It will therefore not be repeated for the latter, so as to avoid a redundancy.

[0057] The method begins with a step 31 consisting in assigning a predetermined initial value to the gain G of the amplifier 15, which normally makes it possible to avoid the saturation of the analog/digital converter 16. A step 32 then consists in comparing a value P_(B) of the power in the converted band with the limit saturation value Psat. If P_(B) is not greater than Psat minus a predetermined margin, we then go to a step 34. If on the contrary P_(B) is greater than Psat minus said margin, then, in a step 33, the gain G of the amplifier 15 is decreased in such a way that the power P_(B) in the converted band is substantially equal to a desired value P_(B)o less than or equal to Psat minus said margin. For this purpose, two cases are distinguished. If P_(B) is less than or equal to P_(B)max, then the current value G of the gain of the amplifier 15 is replaced by G+P_(B)o−P_(B). If on the contrary P_(B) is strictly greater than P_(B)max, then the current value G of the gain of the amplifier 15 is replaced by G−ΔG, where ΔG constitutes a relatively high variation in the gain relative to the gain stepsize of the conversion table (which is 0.5 dB) . For example ΔG equals 2 dB. After step 33 we return to the comparison step 32. In this way, possibly after several iterations of step 33, the gain G of the amplifier 15 is such that the power P_(B) in the converted band is at most substantially equal to the value P_(B)o of the mean power level desired in the converted band.

[0058] Step 34 consists in determining the type of propagation, static or dynamic, in the useful channel as a function of the history of the values P_(C) of the power in the useful channel which are obtained in the absence of saturation of the converter 16. It will be detailed hereinbelow in conjunction with the flow chart of FIG. 5. It is followed by a step 35 of determining a possible “fading hole” in the useful channel. This step 35 will be detailed hereinbelow in conjunction with the flow chart of FIG. 6.

[0059] Next, the method comprises a step 36 consisting in comparing a value P_(C) of the power in the useful channel with the mean power level P_(C)o desired in this channel.

[0060] More exactly, a check verifies whether the value P_(C) lies in the interval around P_(c)o defined by the values P_(C)min and P_(C)max. If P_(C) is not outside this interval, then the end 38 of the method has been reached. Conversely, if P_(C) is outside this interval, then, in a step 37, the gain G of the amplifier 15 is modified so that the value P_(C) is inside said interval. However, this modification of the value of the gain G must not entail any risk of the saturation of the converter 16. This is why a test for validating change of the gain is performed so as to verify that the envisaged new value of the gain does not entail any risk of saturation of the converter 16. The new value of the gain which is envisaged to replace the current value G is for example G+P_(C)o−P_(C). The test for validating the change of gain consists in comparing the value P_(B) of the power in the converted band with the limit saturation value Psat of the converter 16. More exactly, if (P_(C)o−P_(C))+P_(B)≦Psat−margin, then the envisaged new value of gain can be adopted since there is no risk of it causing the saturation of the converter 16. Otherwise, the new value of the gain must be limited to Psat−margin−P_(B). In this way, priority is given to the avoiding of the saturation of the converter 16 over the obtaining of the best possible sensitivity in the useful channel. Stated otherwise the gain of the amplifier is controlled in such a way that the power in the useful channel is substantially equal to the predetermined mean power level P_(C)o. Thus, the gain of the amplifier is increased at most by only a value such that the power in the converted band remains less than the limit saturation value Psat of the converter 16 minus said determined margin.

[0061] After step 37 we are at the end 38 of the method. Nevertheless, steps 32 to 37 may be repeated cyclically both during the initialization phase and during the holding phase.

[0062] Represented in FIG. 5 is a flow chart showing substeps of step 34 of determining the type of propagation in the useful channel.

[0063] During substep 341, the values P_(C)i of the instantaneous power of the useful channel are produced and kept over a duration T. The value of T depends on a speed v of the mobile above which the mobile is regarded as being in the dynamic regime and below which the mobile is regarded as being in the static regime. T must then be greater than the period of the fadings, that is greater than 1/(2*f_(d)), where f_(d) represents the Doppler frequency, given by the expression f_(d)=f*v/c; f represents the carrier frequency of the signal and c the speed of light. To dimension T, the variations of f from one channel to another of one and the same system are disregarded. By way of example, the value of v is fixed at 10 km/h.

[0064] During step 342, the maximum and the minimum of the power values P_(C)i over the time interval T are calculated. These two values can also be calculated using respectively the N largest values (in place of the maximum) or the N smallest values (in place of the minimum). The deviation between these two extreme values will make it possible to update the regime to be considered.

[0065] If the deviation is less than a threshold S₁, this signifies that the power P_(C)i has varied little over the time interval T and that the propagation is of the static type in the useful channel. A substep 343 is then performed which consists in assigning a value corresponding to this type of propagation to the predetermined mean power level P_(C)o. If conversely the difference is greater than the threshold S₁, then this signifies that the power P_(C)i has varied significantly over the time interval T and hence that the propagation is of the dynamic type. A step 344 is then performed which consists in assigning a value corresponding to this type of propagation to the predetermined power level P_(C)o.

[0066] In one example, in step 343 the value Pmin+7 dB is given to Po, that is the predetermined mean power level is 7 dB above the value Pmin of the noise floor of the converter 16. Correspondingly, the value Pmin+4 db is given to the value P_(C)min, this signifying that the acceptable minimum level for the power for the useful channel is situated 4 dB above the value Pmin of the noise floor of the converter 16. Again likewise, the value Pmin+10 dB is given to the value P_(C)max, this signifying that the acceptable maximum value of the power in the useful channel is situated 10 dB above the noise floor Pmin of the converter 16. In step 344, the values Pmin+15 dB, Pmin+12 dB, and Pmin+18 db are given to the values P_(O), P_(C)min and P_(C)max respectively.

[0067] Represented in FIG. 6 is a flow chart showing substeps of step 35 of determining a “fading hole” of the signal in the useful channel.

[0068] During steps 351 and 352, partly merged with step 341 of FIG. 5, two consecutive values P_(C)i and P_(C)i+1 of the power in the useful channel are produced. These two values are instantaneous values of the power in the useful channel. In a step 353, the difference P_(C)i+1−P_(C)i between these two consecutive values is compared with a second determined threshold S2 which is greater than the threshold S1 mentioned hereinabove in conjunction with FIG. 5. If this difference is less than the threshold S2 then this signifies that the signal in the useful channel is not in a “fading hole”. This is why in a step 354, the second value P_(C)i+1 is taken into account in the calculation of the mean values P₁n to P₅n of the power in the useful channel. In the converse case, represented symbolically by substep 355, this signifies that the signal in the useful channel is in a “fading hole”. This is why the second value P_(C)i+1 is not taken into account in the calculation. It is quite simply ignored. 

1. A method of analog/digital conversion of a radiocommunication signal with the aid of an analog/digital converter (16) preceded by a variable-gain amplifier (15), the frequency band of the converted signal containing at least one useful channel, consisting in controlling the value (G) of the gain of the amplifier (15) in such a way that the power in the frequency band of the converted signal is less than a limit saturation value (Psat) of the analog/digital converter (16) and that the power in the useful channel is substantially equal to a desired mean power level (P_(C)o) in the useful channel having a first predetermined value in the case of static propagation in the useful channel or a second predetermined value, different from said first predetermined value, in the case of dynamic propagation in the useful channel.
 2. The method as claimed in claim 1, comprising the steps consisting in: comparing (32) values (P_(B)) of the power in the frequency band of the converted signal with the limit saturation value (Psat) of the analog/digital converter (16); and if a value of the power in the frequency band of the converted signal is greater than said limit saturation value (Psat) of the analog/digital converter minus a predetermined margin, decreasing (33) the gain (G) of the amplifier (15) in such a way that the power in the frequency band of the converted signal is at most substantially equal to a desired mean power level (P_(B)o) in the converted band.
 3. The method as claimed in one of claims 1 or 2, comprising a step (34) consisting in determining the type of propagation, static or dynamic, in the useful channel.
 4. The method as claimed in claim 3, in which the type of propagation in the useful channel is determined as a function of the history of the values (P_(C)) of the power in the useful channel which are obtained in the absence of saturation of the analog/digital converter (16).
 5. The method as claimed in any one of the preceding claims, comprising the steps consisting in: comparing values (P_(C)) of the power in the useful channel with said determined mean power level (P_(C)). if a value (P_(C)) of the power in the useful channel is outside a determined interval (P_(C)min−P_(C)max) around said determined mean power level (P_(C)o), modifying the gain (G) of the amplifier (15) so that the value (P_(C)) of the power in the useful channel is inside said determined interval (P_(C)min−P_(C)max).
 6. The method as claimed in claim 5, in which the gain (G) of the amplifier (15) is controlled in such a way that the power in the useful channel is substantially equal to said determined mean power level (P_(C)o)
 7. The method as claimed in claim 5 or claim 6, in which the gain (G) of the amplifier is increased, at most, only by a value such that the power in the frequency band of the converted signal remains less than the limit saturation value (Psat) of the analog/digital converter (16) minus said determined margin.
 8. The method as claimed in any one of claims 5 to 7, further comprising a step consisting in determining whether the signal in the useful channel is situated in a “fading hole”, the gain of the amplifier being modified, if appropriate, only if the signal in the useful channel is not situated in a “fading hole”.
 9. The method as claimed in claim 2 and/or claim 5, in which the values (P_(B)) of the power in the frequency band of the converted signal and/or the values (P_(C)) in the useful channel which are taken into account in an initialization phase are values (P₁n) of mean power calculated over a time window having a first determined width, while the values (P_(B)) of the power in the frequency band of the converted signal and/or the values (P_(C)) of the power in the useful channel which are taken into account in a holding phase are values (P₅n) of mean power calculated over a time window having a second determined width, greater than said first determined width.
 10. The method as claimed in any one of claims 2, 5 or 9, in which the values (P_(B)) of the power in the frequency band of the converted signal and/or the values (P_(C)) of the power in the useful channel which are taken into account are calculated on the basis of measurements of the instantaneous power (P_(n)) after a first determined lag has elapsed after bringing into service or modifying a parameter of an analog part upstream of the analog/digital converter (16).
 11. The method as claimed in any one of claims 2, 5, 8 or 9, in which the values (P_(B)) of the power in the frequency band of the converted signal and/or the values (P_(C)) of the power in the useful channel which are taken into account are calculated on the basis of measurements of the instantaneous power after a second determined lag has elapsed after the modification gain (G) of the amplifier (15).
 12. The method as claimed in any one of claims 2, 5, 8, 9 or 10, in which the values (P_(B)) of the power in the frequency band of the converted signal and/or the values (P_(C)) of the power in the useful channel which are taken into account are converted into values in decibels independent of the gain of the amplifier by means of a predetermined conversion table.
 13. A device for analog/digital conversion of a radiocommunication signal whose frequency band contains at least one useful channel, comprising an analog/digital converter preceded by a variable-gain amplifier, and means for controlling the value of the gain of the amplifier in such a way that the power in the frequency band of the converted signal is less than a limit saturation value of the analog/digital converter and that the power in the useful channel is substantially equal to a desired mean power level (P_(C)o) having a first predetermined value in the case of static propagation in the useful channel or a second predetermined value, different from said first predetermined value, in the case of dynamic propagation in the useful channel.
 14. The device as claimed in claim 13, comprising: means for comparing values of the power in the frequency band of the converted signal with the limit saturation value of the analog/digital converter; and means for decreasing the gain of the amplifier in such a way that the power in the frequency band of the converted signal is at most substantially equal to said limit value minus a determined margin, if a value of the power in the frequency band of the converted signal is greater than said limit saturation value of the analog/digital converter minus said determined margin.
 15. The device as claimed in one of claims 13 or 14, further comprising means for determining the type of propagation, static or dynamic, in the useful channel as a function of the history of the values of the power in the useful channel which are obtained in the absence of saturation of the analog/digital converter.
 16. The device as claimed in one of claims 13 to 15, further comprising: means for comparing values of the power in the useful channel with said predetermined mean power level; and means for modifying the gain (G) of the amplifier (15) so that the value (P_(C)) of the power in the useful channel is inside a determined interval (P_(C)min−P_(C)max) around, said determined mean power level (P_(C)o), if a value (P_(C)) of the power in the useful channel is outside said determined interval.
 17. The device as claimed in claim 16, further comprising means for determining whether the signal in the useful channel is situated in a “fading hole”, the gain of the amplifier being modified, if appropriate, only if the signal in the useful channel is not situated in a “fading hole”.
 18. The device as claimed in any one of claims 13 to 17, further comprising a first unit for measuring power (18) delivering first values (P₁n) of the power in the frequency band of the converted signal which are values of mean power calculated over a time window having a first determined width, and second values (P_(N)n) of the power in the frequency band of the converted signal which are values of mean power calculated over a time window having a second determined width, greater than said first determined width.
 19. The device as claimed in any one of claims 13 to 18, further comprising a second unit for measuring power (20) delivering first values (P₁n) of the power in the useful channel which are values of mean power calculated over a time window having a first determined width, and second values (P_(N)n) of the power in the useful channel which are values of mean power calculated over a time window having a second determined width, greater than said first determined width.
 20. Device as claimed in claim 18 or claim 19, in which the first (18) and/or the second (20) unit for measuring power comprise: a) means (100, 101) for producing a series of successive samples (Sn) of the signal (S); b) means (102) for producing a series of successive values (Pn) of the instantaneous power of the signal (S), each of these values being obtained from the value of a respective sample of the series of successive samples (Sn) of the signal (S); c) means (103 ₁−103 _(N)) for producing N series of successive values of the mean power of the signal (S) over respectively N time windows of respective increasing widths, where N is an integer such that N≧2, from the values of the series of successive values (Pn) of the instantaneous power of the signal (S).
 21. The device as claimed in any one of the preceding claims, further comprising a management unit (21) connected to the output of the first (18) and/or of the second (20) unit for measuring power so as to receive respectively the values (P_(B)) of the power in the frequency band of the converted signal and/or respectively the values (P_(C)) of the power in the useful channel, and means for compensating for the difference of delay in the transmission of these respective values which is due to the difference of the paths taken, so as to deliver at the input of the management unit (21) values (P_(B)) of the power in the converted band and values of the power (P_(C)) in the useful channel referring to identical samples (Sn) of the converted signal of the converted signal. 